Variable air volume HVAC systems employ a central fan (or "primary supply") system and multiple "terminal units" (also referred to as a "box" or "terminal box") which maintain proper zone conditions by adjusting the amount of airflow to each zone in order to maintain a space temperature setpoint. One example of such a prior art system is disclosed in U.S. Pat. No. 5,005,636 incorporated herein by this reference.
Typically, a variable air volume central fan system comprises a central fan with some means of varying the flow of air from the central fan to the ductwork that supplies air to a network of terminal boxes. Each terminal box regulates the quantity of airflow in an attempt to meet current local space conditions as measured by a local zone temperature sensor. (For simplicity, this discussion assumes that each zone has a single corresponding terminal box.) It is known to use a computer-based or other digital controller to operate each terminal box, and the adjustment of airflow in response to sensed temperature change is the subject of existing patents such as U.S. Pat. Nos. 5,325,286; 5,303,767; and 4,646,964.
Variable volume air systems have been employed for heating and air conditioning in commercial buildings for about twenty-five years. They are currently the system of choice by the industry, and widely employed in office and institutional buildings. In a variable air volume system, one or more central air supply fans are sized to meet the anticipated peak cooling (and/or heating) requirements for the building. Each individual terminal box is sized to meet expected peak conditions of the space (or zone) it serves, which may or may not coincide with building peak conditions.
Each terminal box in a variable air volume system is provided with a preset box maximum airflow level. The box reacts to meet the loads on the space as determined by a space temperature sensor and provides airflow to cool (or heat) the space as needed, but only up to that preset maximum airflow. No further airflow will be delivered no matter how much further the space temperature varies from setpoint conditions. This box maximum airflow level is applied to ensure a reasonable balance of airflow is available to all boxes at all times, even when some zones may be experiencing severe or unusual loads. Adjustment of the terminal box maximum airflow levels is known in trade as "balancing" the HVAC system. In general, each terminal unit operates "open loop" in that the overall load on the primary air supply is unknown and is ignored. As a result, each terminal unit attempts to "take" whatever conditioned airflow volume it deems necessary, and some units may be "starved" if the system is not properly balanced.
Considerable time and effort is required to balance known variable air volume systems at the time of their installation. A trained installer collects airflow and temperature measurement data in each zone, and then attempts to set a respective maximum airflow level for each terminal box such that all boxes have a reasonable airflow level available at all times. Obviously, this procedure represents a compromise in allocating a limited resource, and may not be optimal. User complaints may require another attempt at balancing the system. Moreover, manufacturers recommend rebalancing every few years as the loads in each zone change, for example due to rearrangement of seating and furniture and/or changes in window coverings. Rebalancing therefore is expensive and even if it is well done changing conditions can require it to be done periodically. It is known that a digital network can be employed to adjust terminal dampers in response to one or more zones experiencing air starvation. See U.S. Pat. No. 5,341,988. However, there is no known existing technology that provides automatic system-wide airflow balancing in which box maximum airflow settings are adjusted in response to the central fan conditions as well as local zone conditions. Nor does the prior art teach how to avoid initial air balancing of terminal units at the time of installation. A need remains therefore to reduce the frequency and cost of rebalancing a variable air volume system. Moreover, the need remains to improve the accuracy of balancing such a system so as to maximize user comfort and operating economy.
Another requirement in a variable air volume system is to maintain at least a selected minimum outside air ventilation airflow to each zone whenever the zone is occupied. In some systems, each terminal unit is connected to at least two ducts--a conditioned air duct and an outside air (or "ventilation ") duct. In such systems, each terminal unit determines an appropriate mix of conditioned air together with outside air, based on zone temperature setpoints. Automatic rebalancing must take into account minimum ventilation requirements.